Signatures of the transition from galactic to extragalactic cosmic rays

TL;DR

This paper examines the signatures of the transition from galactic to extragalactic cosmic rays, focusing on the dip scenario caused by pair production in the CMB, and analyzes observable features like spectrum, anisotropy, and composition.

Contribution

It provides a detailed analysis of the dip scenario's signatures, especially in elongation rate and $X_{max}$ distributions, to distinguish it from other models.

Findings

$X_{max}(E)$ shows a sharp increase indicating a transition from iron to proton dominance.

The shape of $X_{max}$ distributions can discriminate between the dip scenario and other models.

The dip scenario offers a simple physical explanation for the cosmic ray transition.

Abstract

We discuss the signatures of the transition from galactic to extragalactic cosmic rays in different scenarios, giving most attention to the dip scenario. The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons in the energy range $1\times 10^{18} - 4\times 10^{19}$ eV, which is caused by electron-positron pair production on the cosmic microwave background (CMB) radiation. The dip scenario provides a simple physical description of the transition from galactic to extragalactic cosmic rays. Here we summarize the signatures of the pair production dip model for the transition, most notably the spectrum, the anisotropy and the chemical composition. The main focus of our work is however on the description of the features that arise in the elongation rate and in the distribution of the depths of shower maximum $X_{\rm max}$ in the dip scenario. We find that the curve for $X_{\max}(E)$ shows a sharp increase with energy, which reflects a sharp transition from an iron dominated flux at low energies to a proton dominated flux at $E\sim 10^{18}$ eV. We also discuss in detail the shape of the $X_{\max}$ distributions for cosmic rays of given energy and demonstrate that this represents a powerful tool to discriminate between the dip scenario and other possible models of the transition.